This invention relates to wafer bonding techniques. More particularly, this invention relates to a system and method for bonding imperfect or out-of-flat wafers together.
Microelectromechanical systems (MEMS) are integrated micro devices which may be fabricated using integrated circuit batch processing techniques. MEMS devices have a variety of applications including sensing, controlling and actuating on a micro scale. Accordingly, MEMS devices often include a moveable component such as a sensor or actuator. Because the MEMS devices are generally moveable, they are also vulnerable to damage from handling or contamination Therefore, the devices are often encapsulated with a lid wafer to protect the moveable component. In addition, some MEMS devices, such as infrared bolometers, require vacuum environment to obtain optimum performance. Accordingly, a lid wafer may need to be sealed against the device wafer with a hermetic bond.
To form the encapsulated device, a lid wafer is aligned to the device wafer, and lid wafer and device wafers are pressed together while heating to cure an adhesive which permanently bonds the wafers in position. The individual devices may then be subsequently singulated, for further packaging with an applications-specific integrated circuit (ASIC), for example. The circuitry of the application-specific integrated circuit may control the functioning of the moveable component of the MEMS device.
If either the lid wafer or the device wafer is out-of-flat or otherwise imperfect, a disproportionate share of the pressure between the wafers is supported on a few points, rather than applied uniformly across the wafer. As a result, bonding may be poor in regions where the pressure is not applied uniformly, which may destroy the hermeticity of the seal, or the adhesion between the wafers. In order to distribute the force more uniformly, a compressible insert may be placed between a pressure-generating chuck and the assembly to be bonded, that is, the device wafer and the lid wafer.
An example of a prior art wafer bonding tool 100 is shown schematically in FIG. 1. The wafer bonding tool 100 includes a pressure chuck 110 which applies a downward bonding force 120 on the lid wafer 140 against a device wafer 150. The device wafer 150 is supported by a wafer chuck 160. A graphite pad 130 may be inserted as the compressible insert between the pressure chuck 110 and the lid wafer 140. The purpose of the graphite pad is to distribute the force 120 over a wider area. Since the graphite pad is compressible, it may become compressed in areas sustaining the maximum pressure, and expanding to fill areas which would otherwise not be in contact.
However, the graphite pad has a number of disadvantageous features. For example, it can support a maximum deformation of on the order of tens of microns, whereas non-uniformities on the order of 50 microns or more may exist between the wafer pair. Therefore, the graphite pad may “bottom out” in a situation wherein 50 microns or so of non-uniformity exists between the wafer surfaces. This situation may result in all of the bonding force being born over a small area, or even at a single point on the wafer, leading to non-uniform bonding over the wafer, or worse, cracking of the wafer, leading to an increased scrap rate.
Furthermore, because the deformation of the graphite pad is not entirely elastic, it does not completely return to its original shape after deformation. As a result, the lifetime of the graphite pads may also be limited to tens of bonding cycles, before its loss of elasticity renders it unusable. In addition, the graphite pad is known to shed debris which may contaminate the package and interfere with the functioning of the MEMS device.